Laser array and method of making same

ABSTRACT

A laser array and method of making same has precision fiducial marks that aid in the alignment of the laser array. The invention requires forming additional optical features adjacent to the laser array that is used to write fiducial marks on an opposite surface in the medium containing the laser array. Fiducial marks are formed when high intensity collimated beams of light are directed through the optical features onto a treated portion of the transparent medium. Fiducial accuracies of 1 micron are possible by using this approach.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 10/028,035, filed Dec. 20,2001 now abandoned.

FIELD OF THE INVENTION

The invention relates generally to the field of microlens lens arrays.More particularly, the invention concerns forming fiducial marks onoptical articles that require precise alignment in an optical systemcontaining the microlens array.

BACKGROUND OF THE INVENTION

Optical systems, such as imaging systems, telecommunications devices,micro-optical systems, micro-mechanical systems, etc., are typicallyconstructed of several different lenses and optical articles to deliverthe desired optical performance. To avoid large overall losses in theoptical system, the alignment of each lens and optical article withsubsequent lenses and optical articles must be very precise. Fiducialmarks are often created on the lenses and optical articles outside theoptical area to serve as a reference point during alignment. Fiducialmarks are particularly important in the case of aspheric lenses and lensarrays where it is difficult to identify the center of the lens duringalignment activities. Fiducial marks are also very important for fiberoptic arrays and laser arrays where multiple features dictate the needfor a shared alignment reference which is located precisely in relationto all the optical features. As optical systems get smaller for fiberoptics applications, like telecommunications and optical sensors, theneed increases for precise alignment of the optical components and theaccuracy of the associated fiducial marks. Alignment specifications oftwo (2) microns are now common with a desire to deliver submicronalignment accuracy. Consequently, the fiducial marks must be locatedwith an accuracy of 1 micron or better.

Fiducial marks are well known in the semi-conductor manufacturingindustry as an important tool for making multilayer semiconductors. Inthis case, the fiducial marks are incorporated as part of thesemiconductor circuit plan. Due to the thinness (50-100 micron) of thesemiconductor layers used in making multilayer semiconductors, thefiducial marks of multiple semiconductor layers can be viewedsimultaneously using a high magnification microscope. The highmagnification microscope aids in positioning the fiducial marks of onesemiconductor layer over the fiducial marks of another semiconductorlayer during the alignment process.

Forming fiducial marks in optical articles raises special challenges inthat optical surfaces are typically relatively thick, often well over a1000 micron in thickness. This is the case even in a microlens arraythat has microlenses that are well under a millimeter in diameter. Thethickness of the microlens array makes it virtually impossible toaccurately locate a fiducial mark by looking through the microlens arraydue to optical limitations. On the one hand, the location accuracy ofthe fiducial mark relative to the optical article is limited because thefiducial mark is displaced by refracted light passing through themicrolens array material. Moreover, the thickness of the microlens arraylimits how close the microscope used for identifying the microlens arraycan be positioned to the fiducial mark. Consequently, only lowermagnification microscopes can be used to look at the fiducial.Therefore, for optical articles, a method of applying a very accuratelylocated fiducial mark on the side opposite to the optical article isneeded.

In U.S. Pat. No. 6,005,294, by Tsuji et al., Dec. 21, 1999, entitled“Method Of Arranging Alignment Marks,” a method of making semiconductordevices uses multiple fiducial marks in such a way that the areaoccupied by the fiducial marks is reduced and the manufacturingproductivity is correspondingly increased. While this patent doesdescribe the state of the art for making semiconductor devices, thealignment process described therein is not appropriate for opticalarticles like lens arrays. As mentioned, in lens arrays, the significantthickness of the various lenses makes it impossible to view fiducialmarks from multiple optical articles simultaneously due to theseparation distance imparted by the material thickness of the lenses.

Also, U.S. Pat. No. 5,850,276, by Ochi et al., Dec. 15, 1998, entitled“Method Of Making LCD Device Having Alignment Mark Made Of Same MaterialAnd Formed At Same Time As Microlenses” and U.S. Pat. No. 5,771,085, byOchi et al., Jun. 23, 1998, entitled “LCD Device With an Alignment MarkHaving Same Material As Microlenses” each describe a process for moldingfiducial marks into a microlens screen used for liquid crystal displaydevices. In these patents the shapes of the fiducial marks are alsodescribed in detail. The fiducial marks as described are protrusions inthe shape of a cross or several other variations, located on the sameside as the microlenses. The protrusions can be semicircular in crosssection or another shape as long as the grooves between the protrusionsstand out as dark lines when viewed with a reflecting microscope. Thereferences recognize that lens characteristics, such as thickness,interfere with the ability to identify underlying fiducial marks.Further, the references show some appreciation for useful geometries offiducial marks and for fiducial marks molded along with a microlensarray. However, neither of the patents show appreciation for fiducialmarks applied on the side opposite the optical surfaces in the microlensarray. Furthermore, there is no appreciation by either of the referencesthat advantages can be gained with a molded fiducial mark having lenscharacteristics.

Moreover, U.S. Pat. No. 6,096,155, by Harden et al., Aug. 1, 2000,entitled “Method Of Dicing Wafer Level Integrated Multiple OpticalElements” discloses the use of fiducials to aid in alignment ofmicrolenses on wafers during the bonding of multiple wafers togetherprior to dicing. This patent generally teaches making integratedmultiple optical elements with features to help control the thickness ofadhesives and solders used to bond together the wafers. While effectiveuse of the fiducial marks is described, there is absolutely no mentionof ways to improve alignment of fiducial marks on one side with theoptical element on the other side of the wafer. The techniques ofembossing and molding fiducial marks, described in the patent, bothsuffer from locational inaccuracies from one side to the other o theorder of plus or minus ten (10) microns. In molded microlenses andmicrolens arrays this inaccuracy is not acceptable.

Furthermore, U.S. Pat. No. 4,598,039, by Fischer et al., Jul. 1, 1986,entitled “Formation Of Features In Optical Material” describes the useof a laser to remove optical material in a controlled fashion. The lasercan be used directly on the optical material or a layer of ablativeabsorber material can be put onto the surface of the optical material toenhance the coupling to the laser. This ablative technique is wellsuited to making fiducial type marks for alignment. However, thereference does not show appreciation for how to align the laser with alens array that is located on the opposite side from the desiredlocation for the fiducial marks.

Therefore, a need persists in the art for a method of forming fiducialmarks onto optical articles and optical arrays on a surface opposite theoptical article surface that enables precise alignment of the articlesand optical arrays. Moreover, there is a compelling need for a specialoptical feature molded along with optical surfaces to focus light ontoan opposing surface of the optical article or optical array thusenabling the formation of a fiducial mark onto the opposing surface withgreat accuracy.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to provide a laser arrayand method in which precise alignment of the laser array is achievedwith fiducial marks formed on a surface opposite the mounting surface ofthe laser array.

It is a further object of the invention to utilize an optical featurethat focuses a high intensity beam of light onto a surface opposite thesurface of the optical feature to thereby form fiducial mark.

To accomplish these and other objects, features and advantages of theinvention, there is provided, in one aspect of the invention, a laserarray having a plurality of lasers supportedly arranged on a firstsurface of a transparent substrate. The transparent substrate has asecond surface opposite the first surface. At least two optical featuresare arranged on the first surface of the transparent substrate adjacentto the plurality of lasers. A fiducial mark precisely corresponding toeach one of the optical features is formed on the second surface of thetransparent substrate.

In another aspect of the invention, a method of manufacturing a laserarray having a plurality of lasers comprises the steps of providing atransparent substrate having a plurality of openings configured forsupporting the lasers. The openings pass at least partially through afirst surface of the transvarent substrate that also has a secondsurface opposite the first surface. The plurality of lasers is arrangedin the openings. At least two optical features are formed on the firstsurface of the transnarent substrate adjacent the laser array. At leasta portion of the second surface of the transnarent substrate is altered.A collimated beam of light is directed through the at least two opticalfeatures and onto the at least portion of the second surface. Theoptical features focuses the collimated beam of light onto the alteredportion of the second surface to form precisely located fiducial marksthereon.

Consequently, the present invention has numerous advantages over priorart developments, including: it results in precision locating offiducial marks; it is a far superior method of aligning optical articlesin an array; and, it is significantly easier to implement since allrequired optical features are formed with the same forming process.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing as well as other objects, features and advantages of thisinvention will become more apparent from the appended Figures, whereinlike reference numerals denote like elements, and wherein:

FIG. 1 a is a perspective view of a prior art lens array with fiducialmarks located on the side opposite the lens surfaces;

FIG. 1 b is a perspective view of an optical system made in accordancewith the method of the invention;

FIG. 2 is an elevated side view of a microlens array set in atransparent medium having optical features formed in accordance with themethod of the invention;

FIG. 3 is an elevated side view of a microlens array set in atransparent medium with a fiducial mark forming means arranged forforming fiducial marks on an opposing surface of the transparent medium;

FIG. 4 is a perspective view of the optical article having generallycircular fiducial marks on a surface opposite the optical article;

FIG. 5 is a perspective view of the optical article of the inventionhaving a generally linear crossed fiducial mark on a surface oppositethe optical article;

FIGS. 6 a and 6 b are perspective views of an alternative embodiment ofthe invention having a plurality of optical articles on either face ofthe transparent medium with corresponding fiducial marks on opposingsurfaces in the transparent medium opposite the optical article;

FIG. 7 is a perspective view of an alternative embodiment of theinvention comprising a laser array; and,

FIG. 8 is a perspective view of another embodiment of the inventioncomprising a fiber optic array.

DETAILED DESCRIPTION OF THE INVENTION

A typical prior art microlens array 2 is illustrated in FIG. 1 a forcomparative purposes. According to FIG. 1 a, microlens array 2 hasmultiple microlenses 1 mounted coincidentally on a mounting flange 3.Fiducial marks 7 are located on a surface 5 of mounting flange 3opposite the surface 6 of microlenses 1. Fiducial marks 7 would eitherbe directly molded onto surface 5 or would be applied after referencingan edge of the optical surface from the opposite side of mounting flange3. In the case of direct molding of the fiducial marks 7, moldmisalignment due to clearance in the alignment pins across the moldedparting line would limit the accuracy of fiducial mark 7 toapproximately 15 microns or more. Using the edge referencing technique,experience has taught that each measurement introduces approximately 2-5microns of inaccuracy. Since a minimum of three (3) measurements arerequired to identify an edge of a round lens, the total inaccuracy is aminimum of 6-15 microns to place the fiducial mark 7. While thisinaccuracy is usually acceptable for large optical articles, as the sizeof optics for applications such as fiber optics shrinks below 1000micron, the alignment accuracy required shrinks as well. Consequently,it is not uncommon for alignment accuracy of microlenses to be 5 micronsor better with some applications calling for 2 micron alignment.Obviously, the accuracy of the fiducial marks 7 must be better than thealignment accuracy required.

Turning now to FIG. 1 b, fiducial marks 13 formed in an optical articlearray, such as refractive lens array 11, using the method of theinvention is illustrated. In this embodiment, fiducial marks 13 are usedto align an optical assemblage 8 comprising refractive lens array 11 andlaser array 9. As described in the invention, additional opticalfeatures 16 are used to create fiducial marks 13 through optical means.According to FIG. 1 b, fiducial marks 13 on the lens array 11 areprecisely located on opposing surface 11 b of lens array 11. To ensureprecise alignment of optical assemblage 8, each one of a plurality ofprecision through-holes 15 formed in laser array 9 is alignably centeredover a corresponding fiducial mark 13 in lens array 11. This processaligns each of the lasers 9 a in the laser array 9 with a refractivelens 11 a in the refractive lens array 11. After the optical assemblage8 is aligned, it is rigidly affixed typically by potting in a suitableadhesive material. Precise alignment of precision through-holes 15 overthe fiducial marks 13 is accomplished with a high power microscope (notshown) often with a computerized vision system linked to a computerizedpositioning system to automate the process.

Referring to FIGS. 2 and 3, an optical array 10 having accuratelylocated fiducial marks 24, 28 formed on an opposing surface 30 of atransparent substrate 12 is illustrated. According to FIGS. 2 and 3,optical articles, such as microlens array 22, 32, are supported onmounting surface 14 of transparent substrate 12 that is opposite surface30. Important to the invention, an additional optical feature 20(described below) is formed adjacent to the microlens array 22, 32 toaid in precisely forming fiducial marks 25, 29 at focal points 24, 28.According to FIG. 2, focal point 24 (corresponding to a fiducial mark25) is then produced with a high intensity collimated beam of light 26.As shown in FIG. 3, a laser source 27 may be used to produce such highintensity light 26. The additional optical feature 20 receives thecollimated beam of light 26 from laser source 27 and precisely focusesit 28 onto opposing surface 30 of the microlens array 10. It is alsoimportant to the invention that prior to forming the fiducial marks 25,29 at focal points 24, 28, surface 30 of the transparent substrate 12 isaltered or treated in the area 31 where the fiducial marks 25, 29 are tobe formed. The objective of altering or treating surface 30 is to makesuitably visible fiducial marks 25, 29 when exposed to the focused highintensity light 26. Suitable surface altering techniques include dipcoating, roughening, spin coating, vacuum coating, metallizing, amongothers.

Skilled artisans will appreciate that there are several processes thatmay be used for forming a mold for making optical articles, such asoptical array 10, which includes additional optical features 20 asdescribed. Such processes include lithographic printing, ink jetprinting, indentation, diamond turning and diamond milling, each ofwhich can deliver a position to position accuracy of 0.25 micron.Importantly, the method of the present invention uniquely uses theprocess for forming the microlens array 32 for also forming theadditional optical features 20 that precisely locates the fiducial marks25, 29 at focal points 24, 28.

Referring to FIGS. 4 and 5, optical features having a variety ofconfigurations with refractive or diffractive lenses can be used tocreate various shaped fiducial marks. According to FIG. 4, a lens array40 has a plurality of lenses 41 formed on first surface 46 oftransparent medium 44. Generally round refractive lens feature 45 can beused to make a generally round fiducial mark 42 on the treated portion49 of second surface 48 of transparent medium 44, opposite first surface46 of the transparent medium 44. Moreover, to produce a generally linearfiducial mark, a generally linear lens feature is required (not shown).According to FIG. 5, a generally crossed linear refractive lens feature50 is used to produce a generally crossed-shaped (X-shaped) fiducialmark 52. Those skilled in the art will now appreciate that otherpatterns for the optical feature can be produced by a combination ofrefractive and diffractive optical features.

Referring to FIGS. 6 a and 6 b, in another embodiment of the invention,double-sided optical arrays 58, 59 are illustrated. According to FIG. 6a, double-sided optical array 58 has an arrangement of optical articles60, 62 on either of opposing surfaces 61 a, 61 b in transparent medium61. Fiducial marks 69, 66 are formed on the treated portions 63, 64 ofthe opposing surfaces 61 a, 61 b, respectively, by repeating thefiducial marking process described hereinabove. According to FIG. 6 b,alternatively, double-sided optical array 59 has optical features 72, 80mounted on opposing surfaces 70 a, 70 b of transparent medium 70. Inthis embodiment, two sets of fiducial marks 78, 83 are formed only onthe treated section 76 of surface 70 b opposite surface 70 a so themisalignment between the two optical articles 72, 80 could be easilydetermined.

Referring again to FIG. 6 a, double-sided optical array 58, moreparticularly, has a first plurality of lenses 60 matched to a secondplurality of lenses 62, both being mounted on opposing surfaces 61 a, 61b of transparent medium 61. Two complimentary sets of additional opticalfeatures 65, 68 are formed in either of opposing surfaces 61 a, 61 b,respectively. Optical features 65, 68 are used to form fiducial marks66, 69 on the opposing surfaces 61 b, 61 a, respectively. As shown inFIG. 6 a, optical feature 65 has a generally round shape which forms agenerally round shaped fiducial mark 66 on the opposing surface 61 b. Inthe same alternative, double-sided optical array 58, a generally ringshaped optical feature 68 formed on surface 61 b produces a generallyring shaped fiducial mark 69. Alternatively, fiducial marks 66, 69 andoptical features 68, 65 can be used as matching reference marks tomeasure the relative alignment of the optical articles 60, 62 onsurfaces 61 a, 61 b by measuring the relative centering of the fiducialmarks 66, 69 from the optical features 65, 68.

It is the experience of the inventors that by using both refractive anddiffractive lenses in the additional optical lens features, a widevariety of fiducial mark shapes can be created to fit differentrequirements. The additional optical lens feature can also be designedfor different wavelengths if the fiducial marking is to be done using alight source that operates at a different wavelength than used by theoptical array.

Referring again to FIG. 6 b, another embodiment of a double-sidedoptical array 59 is illustrated. As described above, a first pluralityof lenses 72 in optical array 59 has additional generally round opticalfeatures 74 formed on surface 70 a of transparent substrate 70. Opticalfeatures 74 provide precise focusing of the collimated beam of light(FIG. 2) onto opposing surface 70 b which forms a generally roundfiducial mark 78 on a treated portion 76 of opposing surface 70 b. Inthis embodiment, the second plurality of lenses 80 is formed on opposingsurface 70 b of transparent medium 70. Further, generally squarefiducial marks 83 have been produced along with lenses 80 on the treatedportion 76 of surface 70 b. The alignment of the first plurality oflenses 72 to the second plurality of lenses 80 is preferably determinedby measuring the magnitude and direction of the de-centering, i.e., thedistance from an imaginary centerline passing through the lenses to thefiducial mark of fiducial mark 78 to fiducial mark 83.

In FIGS. 7 and 8, two additional embodiments of the invention areillustrated. According to FIG. 7, a laser array 110, having lasers 90,includes two additional optical features or crossed linear lenses 92that produce fiducial marks 94 in the form of a cross (X) on the treatedportion 97 of opposing surface 96 b. Lasers 90 may be arranged inopenings in transparent medium 96 or they may be bonded to first surface96 a of transparent medium 96. According to FIG. 8, a fiber optic array120, having fiber optic units 100 formed in transparent substrate 106,includes additional optical features 102 adjacent to fiber optic units100 that are used to produce fiducial marks 104 on the treated portion107 of opposing surface 106 b of the fiber optic array 120. The fiberoptic units 100 may be formed in transparent substrate 106 or they maybe bonded to first surface 106 a. The same process, described above, forforming fiducial marks 94, 104, is used in the present embodiments ofthe invention.

The invention has been described with reference to various embodimentsthereof. However, it will be appreciated that variations andmodifications can be effected by a person of ordinary skill in the artwithout departing from the scope of the invention.

Parts List

-   1 microlens-   2 prior art microlens array-   3 mounting flange-   5 surface of mounting flange 3-   6 surface of mounting flange 3 supporting microlens 1-   7 fiducial marks on opposing surface 5-   8 optical assemblage-   9 laser array-   9 a lasers in laser array 9-   10 optical array-   11 refractive lens array-   11 a refractive lens in refractive lens array 11-   11 b surface for fiducial marks-   12 transparent substrate-   13 fiducial marks for refractive lens array 11-   14 mounting surface-   15 precision through-holes-   16 additional optical features used to form fiducial marks 13-   20 additional optical feature-   22 microlens array-   24 focal point of additional optical feature 20 on an opposite    surface to the microlens array-   25 fiducial mark-   26 high intensity collimated beam of light-   27 laser source-   28 focal point produced by the collimated light 26 passing through    the additional optical features 20-   29 fiducial mark-   30 fiducial marking area on the opposite side of the microlens array    22-   31 treated area on surface 30-   32 multiple lens refractive lens array-   40 lens array-   41 plurality of lenses-   42 generally round fiducial mark-   44 transparent medium-   45 generally round refractive lens feature-   46 first surface of transparent medium 44-   48 second surface of transparent medium 44-   49 treated area of surface 48-   50 crossed linear refractive lens feature-   52 cross-shaped fiducial mark-   58 alternative double-sided optical array-   59 alternative double-sided optical array-   60 optical articles (first plurality of lenses in optical array 58)-   61 transparent medium-   61 a, b opposing surfaces in transparent medium 61-   62 optical articles (second plurality of lenses in optical array 58)-   63 treated portion of surface 61 a-   64 treated portion of surface 61 b-   65 round upper additional optical feature-   66 found fiducial mark-   68 ring-shaped lower additional optical feature-   69 ring-shaped fiducial mark-   70 transparent medium-   70 a, b opposing surfaces in transparent medium 70-   72 optical features (first plurality of lenses in lens array 59)-   74 round additional optical feature-   76 treated portion of opposing surface 70 b-   78 round spot fiducial marks-   80 optical features (second plurality of lenses in lens array 59)-   83 square fiducial mark-   90 lasers-   92 crossed linear lens on laser array 110-   94 X-shaped fiducial marks on lower surface-   96 transparent medium of laser array 110-   96 a first surface of transparent medium 96-   96 b second surface of transparent medium 96-   97 treated portion of surface 96 b-   100 fiber optic units-   102 additional optical feature-   104 X-shaped fiducial marks on lower surface-   106 transparent medium of fiber optic array 120-   106 a first surface of transparent medium 106-   106 b opposing surface of transparent medium 106-   107 treated portion of surface 106 b-   110 laser array-   120 fiber optic array

1. Method of manufacturing a laser array having a plurality of lasers,comprising the steps of: providing a transparent substrate having aplurality of openings, each one of said plurality of openings passing atleast partially through a first surface of said transparent substrate,said transparent substrate having a second surface opposite said firstsurface, and each of said plurality of openings being configured tosupport one laser of said plurality of lasers of said laser array;arranging each one of said plurality of lasers into an associated one ofsaid plurality of openings for directing light toward said secondsurface; forming at least two optical features on said first surface ofsaid transparent substrate adjacent said laser array; altering at leasta portion of said second surface of said transparent substrate; andaltering at least a portion of said second surface of said transparentsubstrate; and, directing a collimated beam of light through said atleast two optical features and onto said at least portion of said secondsurface, said at least two optical features focusing said collimatedbeam of light onto said at least a portion of said second surface toform precisely located at least two fiducial marks thereon.
 2. Themethod recited in claim 1 wherein said collimated beam of light isproduced by a laser.
 3. The method recited in claim 1 wherein said atleast two optical features each has a predetermined configuration. 4.The method recited in claim 3 wherein said predetermined configurationis generally circular thereby forming a generally circular fiducial markon said second surface.
 5. The method recited in claim 3 wherein saidpredetermined configuration is generally linear thereby forming agenerally linear fiducial mark on said second surface.
 6. The methodrecited in claim 3 wherein said predetermined configuration is generallycrossed linear thereby forming a generally crossed linear fiducial markon said second surface.
 7. The method recited in claim 1 wherein saidstep of altering includes the step of ablating said at least portion ofsaid second surface so as to distinguish said fiducial mark formedthereon.
 8. The method recited in claim 1 wherein said step of formingsaid at least two optical features includes the step of forming said atleast two optical features by diamond milling.
 9. The method recited inclaim 1 wherein said step of forming said at least two optical featuresincludes the step of forming said at least two optical features bydiamond turning.
 10. The method recited in claim 1 wherein said step offorming said at least two optical features includes the step ofindentation.
 11. The method recited in claim 3 wherein saidpredetermined configuration of said at least two optical featurescomprise generally a diffractive element thereby producing a generallycomplex-shaped at least two fiducial marks.
 12. The method recited inclaim 1 wherein said step of altering includes the step of painting saidat least a portion of said second surface so as to distinguish saidfiducial mark formed thereon.
 13. The method recited in claim 1 whereinsaid step of altering includes the step of metallizing said at least aportion of said second surface so as to distinguish said fiducial markformed thereon.
 14. The method recited in claim 1 wherein said step ofaltering includes the step of vacuum coating said at least a portion ofsaid second surface so as to distinguish said fiducial mark formedthereon.
 15. The method recited in claim 1 wherein said step of alteringincludes the step of roughening said at least a portion of said secondsurface so as to distinguish said fiducial mark formed thereon.
 16. Themethod recited in claim 1 wherein said step of altering includes thestep of spin coating said at least a portion of said second surface soas to distinguish said fiducial mark formed thereon.
 17. The methodrecited in claim 1 wherein said step of altering includes the step ofdip coating said at least a portion of said second surface so as todistinguish said fiducial mark formed thereon.
 18. The method recited inclaim 1 wherein said step of forming said at least two optical featureson said first surface includes the step of diamond milling.
 19. Themethod recited in claim 1 wherein said step of forming said at least twooptical features on said first surface includes the step of diamondturning.
 20. The method recited in claim 1 wherein said step of formingsaid at least two optical features on said first surface includes thestep of lithographic printing.
 21. The method recited in claim 1 whereinsaid step of forming said at least two optical features on said firstsurface includes the step of ink jet printing.